They turn around the cars pretty quick at most airports. It's basically a wash and a quick vacuum, and unless the cleaning crew notices something really amiss like a stain or smell or something, it goes back out on the lot for the next customer. It's highly unlikely that they would check between the seats and anywhere that's not immediately visible. That's probably not what you want to hear, but it's reality. Your idea of a "proper cleaning" and Hertz's are probably quite different.
All of that said, you can certainly complain to a local manager upon return, but there's really nothing you can do prior to that (or at least nothing that is likely to get any results). You'll either have to budget a bit of extra time at return or just complain later via email and hope that they're willing to do something after the fact. Not sure what you can expect in terms of compensation, if any. It's really up to a local manager.